|Year : 2021 | Volume
| Issue : 1 | Page : 5-9
Anatomical and Clinical Characteristics of Paediatric and Adult Eyes
Mittal Sunita1, Sharma Manisha2, Mittal Kumar Sanjeev3, Kumar Satish Ravi4, Juneja Aarzoo5, Agrawal Ajai6
1 Additional Professor, Department of Physiology, AIIMS, Rishikesh, Uttarakhand, India
2 Senior Resident, Department of Ophthalmology, AIIMS, Rishikesh, Uttarakhand, India
3 Prof. and Head, Department of Ophthalmology, AIIMS, Rishikesh, Uttarakhand, India
4 Additional Professor, Department of Anatomy, AIIMS, Rishikesh, Uttarakhand, India
5 Junior Resident, Department of Ophthalmology, AIIMS, Rishikesh, Uttarakhand, India
6 Additional Professor, Department of Ophthalmology, AIIMS, Rishikesh, Uttarakhand, India
|Date of Submission||23-Aug-2020|
|Date of Decision||16-Sep-2020|
|Date of Acceptance||06-Dec-2020|
|Date of Web Publication||27-Jan-2021|
Department of Ophthalmology, AIIMS, Rishikesh, Uttarakhand
Source of Support: None, Conflict of Interest: None
The human eye is one of the special sensory organs. Eyes undergo a series of changes right from infancy to attainment of adulthood. Major changes occur in globe dimensions, orbital and neurological structures. In order to have a sharp focus of the image on the retina, the intraocular structures undergo developmental changes in the 1st year of life along with the neurological growth which enables processing of that retinal image. The extraocular growth of the bony orbit and adnexa accompanies intraocular growth. An ophthalmologist looking after the pediatric patient must be aware of these physiological changes so as to not diagnose them as any pathologic conditions. There are few diseases which can also interfere with these normal developmental changes. It is valuable to know the developmental process to diagnose children with eye disorders. This review article lays emphasis on the normal anatomical alterations in the globe, anterior segment, pupil, lens, retina, lacrimal apparatus, and external orbit of the human eye during infancy and early adulthood and also discussing its practical implications.
Keywords: Adult eye, clinical implications, infantile eye, ocular parameters, pediatric eye
|How to cite this article:|
Sunita M, Manisha S, Sanjeev MK, Ravi KS, Aarzoo J, Ajai A. Anatomical and Clinical Characteristics of Paediatric and Adult Eyes. Natl J Clin Anat 2021;10:5-9
|How to cite this URL:|
Sunita M, Manisha S, Sanjeev MK, Ravi KS, Aarzoo J, Ajai A. Anatomical and Clinical Characteristics of Paediatric and Adult Eyes. Natl J Clin Anat [serial online] 2021 [cited 2021 Jun 13];10:5-9. Available from: http://www.njca.info/text.asp?2021/10/1/5/308112
| Introduction|| |
Eye undergoes marked anatomical and physiological developmental changes in terms of axial length (AL), shape of the cornea, and retinal and neurological changes from infancy to the attainment of adulthood. This review article lays emphasis on the anatomical alterations in the eye and orbit during infancy and early adulthood and their practical implications. Externally, bony growth of the orbit is influenced by the development of the eye, whereas intraocularly, the anterior segment and retina go through quick alterations. Neurologic development of a child also subsequently helps in processing of the retinal images.
One should be familiar with the normal developmental changes in a child's eye, so as to not misunderstand them with any pathology. This review article compares the normal dimensions of the eye at birth with that of an adult and its practical implications. Ocular structure-wise physioanatomical changes are discussed with their applied aspect as below.
| Globe Dimensions|| |
Pediatric eyes are different from adult eyes in many ways. As the child grows, the weight of the eyeball almost doubles ranging from 2.3 to 3.4 g at birth to 7.5 g in an adult. The capacity of an infant globe varies between 2.20 and 3.25 cm3. The AL is the distance from the anterior surface of the cornea up to an interference peak corresponding to the retinal pigment epithelium (RPE)/Bruch's membrane., The average AL of a newborn eye is 16.5 mm. Change in the AL occurs in three phases. The first phase (birth to 2 years) is a period of rapid growth. In an adult, AL becomes ten times as compared to that of an infant. During the second (2–5 years) and third phases (5–13 years), growth slows down, thereafter the rate further slows down to almost 0.4 mm/year.
The increase in the AL dimensions varies in pathological states, such as congenital glaucoma, congenital cataracts, and retinopathy of prematurity. AL monitoring is important for the control of intraocular pressure in congenital glaucoma.
| Cornea|| |
The cornea and sclera together form the outer coat of an eyeball, which acts as a structural barrier against infections. The horizontal corneal diameter is the distance between the nasal and temporal imaginary limbal tangents to the corneal circumference. At birth, the horizontal corneal diameter is about 9.8 mm on an average, while the vertical diameter is 9.9–10.5 mm. The average corneal diameter in adults is 11 mm horizontally and 12 mm vertically. Their significance is in the diagnosis of various pathologies such as microphthalmos, corneal dystrophy, microcornea, and megalocornea. The progress in corneal diameter goes along with changes in corneal curvature. The anterior corneal surface is convex and aspherical and transversely oval as a result of sclerization both superiorly and inferiorly. A study conducted by Zadnik et al. and Mohd-Ali et al. reported that the curvature of cornea steepens with age and Hayashi et al. also proposed that the alteration in elasticity is the underlying cause for this physiological phenomenon. Values attained from keratometer for infants at term vary from 48.06 to 47.00 D. These readings are also sharply reduced in the first 6 months at a pace of −0.40 D/month, following a speed of −0.14 D/month in subsequent 6 months and further −0.08 D/month in the following year of life, attaining adult range at almost 3 years of age.
In a study conducted on 4881 children aged between 6 and 14 years, the above-mentioned values remained stable in the horizontal meridian during childhood, but the readings diminished marginally with age in the vertical meridian. Adult cornea appears to be elliptical due to higher power in horizontal meridian predisposing young adults to with-the-rule (WTR) astigmatism. The flattening of the cornea continues to happen into the second and early third decades. By the age of 40–50 years, the horizontal meridian begins to steepen.
This marks a steady change from WTR astigmatism which is usually present in adolescence to Agaist the rule (ATR) astigmatism which is more prevalent in 50- to 60-year-old individuals [Table 1]., Central corneal thickness (CCT) influences the measurement of intraocular pressure (IOP)., Average CCT at birth is nearly 564 μ which decreases during the first 6–12 months of life, it then increases from approximately 553 μ at 1 year to about 573 μm by 12 years of age and stabilizes thereafter. General anesthesia induces decrease in IOP, however crying leads to increase in IOP in an infant. A study conducted by Ehlers et al. noted that CCT in newborn further reduces and reaches the value of an adult cornea by the age of 3 years. A study by Hahn et al. recommended that reduction in keratocytes decrease in density of keratocytes with age is responsible for the reduction of CCT value with age.
Increase in the corneal thickness might lead to miscalculated intraocular pressure. Mode of measurement also significantly impacts its readings, for example, as per some studies,, intraocular pressure is underestimated both under general anesthesia and in childhood when done by applanation tonometry.
Pediatric keratoplasty is a very challenging procedure and difficulties occurring in such cases consist of challenging preoperative evaluation, intraoperative problems due to low scleral rigidity, increased fibrin reaction, and positive vitreous pressure. In addition, need for regular checkup under anesthesia for postoperative follow-up, loosening of sutures requiring replacement/early removal, increased risk of rejection due to more active immune system; complexities to deal with refractive surprises and reversal of amblyopia make keratoplasty in the pediatric population a difficult procedure.,
| Iris, Pupil, Anterior Chamber Depth, and Angle|| |
The development of crypts present in the iris of the fetus starts during pregnancy and continues through the early postnatal period. Most changes in iris color occur over the first 6–12 months of life, as pigment accumulates in the iris stroma in the form of melanocytes.
Dilator and sphincter muscles of the iris are innervated by the sympathetic and parasympathetic nerves, respectively which further helps in regulating the pupil size. Compared with the adult pupil, the infant pupil is relatively small. A pupil diameter <1.8 mm or >5.4 mm is suggestive of an abnormality.
The depth of the anterior chamber is affected by the development of the sclera, also by the change in the lens movement and its width. The average depth is 2.05 mm with an extent of 1.8–2.4 mm at birth which continues to rise until the end of teenage years, which further decreases with age. The development of the chamber angle in children starts from birth till 2 years of age; however, angle recess appears at 4–5 years of age.
During surgery for glaucoma, the complications are more likely to occur in children than in adults, largely due to the anatomical factors related to ocular enlargement and the inherent characteristics of the pediatric eye. The thinned, stretched sclera of the buphthalmic eye, combined with the elasticity and low scleral rigidity of a pediatric eye make these eyes prone to complications, especially hypotony. The aggressive healing response also hampers the efficiency of surgeries by creating alternative pathways for aqueous drainage from the eye leading to another major challenge.
| Lens|| |
Children have small, soft poorly developed eyeballs, with greater elasticity of the anterior capsule, low scleral rigidity, and elevated vitreous pressure. Intraoperative problems together with high risk for postoperative inflammation, altered postoperative refractive state, complex re-surgery, and an innate possibility of amblyopia; all of these make cataract surgery more complex among the younger age group.
The essential period of anatomical and physiological development is up to 6 months of age and an emmetropic state is achieved at 9 years of age normally but can go beyond this age as well. As the lens flattens, the power is decreased by 8 D in the initial 2 years, which is further decreased by approximately 11.5 D and reaches an adult value between 7 and 10 years. Thus, the visual system is very dynamic before 10 years of age, and calculating the precise power for the child's eye is a challenging task. Intraocular lens (IOL) power calculation in children depends on multiple factors including the age at which the cataract appears, visual acuity at the time of presentation, stage of cataract formation (congenital/developmental), involvement of one or both eyes, and the refractive condition of the opposite eye. As ocular parameters are used to calculate IOL power during cataract surgery, the refractive result after IOL implantation is kept as moderate hypermetropia so as to avoid the myopic shift in later life. Dahan and Drusedau recommended under correction of original power by 20% in children <2 years of age and of 10% in children between 2 and 8 years of age. Due to the hydrophobic property of lenses with square edges, it inhibits the migration of epithelial cells and thus prevents posterior capsule opacification; therefore, it is well suited in the pediatric age group. Posterior capsulorhexis is done for all children of <6 years of age due to high rate of visual axis opacification (VAO).
| Sclera|| |
The sclera is largely made of collagenous material. Its thickness ranges between 0.45 mm in neonates and 1.09 mm in adults. At the limbus, it varies from 0.53 ± 0.14, 0.39 ± 0.17 mm at the equator, and around 1.0 mm near the optic nerve. The collagen present in the sclera undergoes developmental changes in the initial postnatal period. In infants, the sclera is much more flexible as compared to adults and the tensile strength is approximately half. This is depicted by the finding of buphthalmos seen in infantile glaucoma with raised intraocular pressure.
| Retina|| |
During embryogenesis, the retina is formed from the walls of the optic cup. The external layer of the optic cup forms the RPE and its inner layer differentiates into the neuroretina. The entire process of foveal maturation is not complete until 4 years postnatally. The significant postnatal development of the fovea has implications for the sensitivity in this period to amblyogenic conditions.
The most important anatomical concern in pediatric retinal surgery is comparatively smaller size of the globe and orbit in comparison to that of an adult. The ciliary body of the eye has two parts – pars plicata, which in a mature infant is of the same size as that of an adult, the other part is pars plana ciliaris, which is relatively smaller in mature newborn as compared to an adult. In neonates, pars plicata route for vitrectomy is chosen, but its proximity to the lens is one of the major limitations leading to restricted manipulations. Scleral buckle in retinal detachment surgeries induces the refractive error of around −2.75D in adults, but this anisomyopia might be greater in pediatric eyes due to its axial elongation which is caused by encircling band. Hence, to prevent high refractive state, the encircling sclera band in pediatric patients needs to be cut once the desired effect is seen, thus reducing the refractive power.
| Orbital Structures|| |
The development of orbit seems to follow a biphasic pattern. In the initial phase, the development of the orbital bones takes place during the first 3 years of life, thus being the most crucial period. During the second phase, orbital development is closely related to the process of sinus pneumatization. The linear and angular measurements of the orbit are under constant changes from childhood to adolescence and further to adulthood. There are also significant differences between male and female orbits in all age groups. The orbital volume at birth is 10.3 ml and that of an adult is 30 ml. The orbital margin in the newborn is ellipsoid in shape, whereas in adults, it is quadrilateral in shape. This orbital border is sharp and well ossified at birth; thus, the eyeball is well protected from stress and damage all throughout the parturition. The roof of the orbit is larger than its floor at birth, which clinically corresponds to the shape of a fetal skull, which has a large cranium (orbital roof) and small face (orbital floor). Due to small interorbital distance in children, it gives the appearance of a pseudo-squint, i.e., the eyes look too close together, and further with the growth of the frontal and ethmoidal air cells, the interorbital distance increases, and so with time the pseudo-squint disappears.
Because of dissimilar anatomical and mechanical properties of the orbital bones in children, their fractures carry distinctive features. Due to high extent of cancellous bone and budding skull sutures, it allows pediatric facial bones to absorb more energy during a bearing without ensuing in fracture. Because of this in young children, orbital floor fractures are relatively less common, and most orbital fractures involve orbital roof.
| Lacrimal Apparatus|| |
Congenital nasolacrimal duct (NLD) obstruction is extremely a widespread disease, affects 20% of infants, which results from incomplete canalization of the NLD during its embryological development. However, 90% of such obstructions resolve spontaneously. Surgical interventions are indicated only when there is no improvement with the conservative treatment like probing, or it has multiple recurrences. Dacryocystorhinostomy (DCR) involves the formation of a fresh opening between the lacrimal sac and nasal cavity which can be performed through external approach. An osteotomy created through nasal bone and flaps are made involving lacrimal sac and nasal mucosa. Endoscopic DCR is a newer modality, in which the ostium is made by a laser. Pediatric endoscopic DCR is more challenging because of the narrow nasal space. The bony ostium is made to expose the entire sac, but the ostium made in this is smaller; approximately 10 × 10 mm in children and 10 × 15 mm high in older children.
| Extraocular Motility and Visual Acuity|| |
The muscles of the eye are very thin at birth. The distance of insertion of muscles from the limbus at birth is 2 mm less than that in adults. The distance is reduced to <1 mm by the age of 6–9 months and reaches to that of an adult by 20 months. During squint surgery, one or more of the eye muscles are strengthened, weakened, or moved to a different position to improve its alignment.
Visual acuity in a newborn is not fully developed as compared to an adult. The vision of neonates ranges from 6/240 to 6/60 (20/800-20/200). At the age of 2 months, it improves to 6/45 (20/150). In the next 2 months, it develops further by a factor of 2–around 6/18 (20/60). By 6 months of age, vision goes up to the level of 20/20 as that of an normal adult [Table 2].
| Neurologic Development|| |
The development of the neurologic connections to the eye plays a vital role in visual function. Along with visual acuity, the contrast sensitivity, stereopsis, scotopic, and photopic sensitivity are also reduced by periods of visual deprivation during infancy. The earlier in infancy that visual deprivation occurs, the more profound is the resultant decrease in contrast sensitivity. In the first week of life, infants only appreciate shades of gray because of the immature nerve cells in the retina and the brain. In the early years of life, there is a gradual improvement in color sensitivity due to subsequent cone reinforcement. A lot of work has been done in relation to find the precise age at which infants are able to appreciate different colors/chromatic stimuli as a result of significant color factors for example brightness/luminance, saturation, and hue.
Stereopsis, which also requires binocular function, has been shown to decrease with early visual deprivation in one eye. It could be due to decrease in the population of binocularly driven cortical cells or unidentified factors associated with stereopsis.
Each eye perceives a stimulus at a different angle forming two images that are processed further by the brain. This gives the pictorial information with the three-dimensional description of the outside world. Hence, an infant must be able to move eyes and converge them on an object for developing the element of depth perception.
The infants have raised threshold for light sensitivity in comparison to adults. It has to be fifty times higher so as to be appreciated by a 1-month-old infant. Around 2 months, only ten times higher is sufficient. This occurs due to the enlargement of the photoreceptors and additional growth of the retina. Maturation of the retina after birth leads to robust light adaptations for infants.
| Conclusion|| |
To summarize, there is a myriad of changes in ocular features during infancy and early childhood. Ocular development proceeds in tandem with the refractive and neurologic changes to result in the growth of orbital features, in addition to the intraocular elements. Understanding of these normal changes is vital to the care and management of pediatric patients with eye disorders.
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[Table 1], [Table 2]